A traveling wave structure is widely used in high speed linear modulators with Mach-Zehnder modulator (MZM) structures with traditional material platforms such as LiNbO3 and InP. Silicon-on-insulator (SOI) platform, which can be used to make optical waveguide, has appeared as the emerging technology for optoelectronics devices including MZM modulators since SOI fabrication infrastructure is compatible with CMOS technology and is suitable for photonic integration; however, the most promising technology for SOI based optical modulation, which uses carrier depletion through doped P-N junction, suffers high optical loss and high Vpi, where Vpi is defined as the voltage required to generate Pi phase shift on MZM arms. High Vpi requires high driver voltage and thus high power consumption for modulator drivers, which limit to their applications where low power consumptions are required. In additional, high speed modulators are required to meet the ever-increasing bandwidth requirements for today's communication and data center interconnect applications.
An MZM modulator is formed by splitting input optical waveguide into two optical waveguide arms that operate as phase shifters due to electro-optic coupling, which are then combined to form a combined interference signal based on the Mach-Zehnder interferometer structure. By modulating phases on phase shifters, the optical phase and amplitude modulations can be achieved. The most promising method for silicon base optical modulators is to use carrier depletion which consists of a P-N junction inside an optical waveguide. Under the reverse bias condition, the P-N junction depletes carrier and causes a change in refractive index and the phase change as light propagates through the waveguide with the refractive index change. In traveling wave structure based modulators, a phase shifter has overlapped RF and optical waveguides, which can realize electrical-to-optical (E-O) conversion, characterized as phase change (degree) on optical wave propagated along the phase shifter per unit voltage applied to phase shifter. This structure is attractive for high speed linear modulators.